1. Field of the Invention
The present invention relates to ring laser gyroscopes and more particularly, to an improved cavity length controller for a ring laser gyroscope.
2. Description of the Prior Art
A ring laser gyroscope (RLG) is a gyroscope which utilizes a laser beam directed to travel in a closed path to detect rotation about the axis of the path around which the laser beam is directed. The ring laser gyroscope must be capable of operating over a wide range of temperatures. Because of temperature variations, the material which the gyroscope is made of experiences thermal expansion and contraction. The laser beam within the ring laser gyroscope is directed in its path by means of mirrors, however, temperature variations cause a change in the path length. This change in path length, if not corrected, can result in drift and scale factor changes.
U S. Pat. No. 4,386,853 to Ljung discloses a bending actuator for a ring laser gyroscope. This invention corrects bias errors caused by frame distortion by bending the ring laser gyroscope block in an asymetric fashion. The present invention however utilizes symetrically applied force to keep the path length of the laser frame constant during temperature excursions. The concept of the present invention becomes more important as the size of ring laser gyros decrease, which has been the trend over the last decade. U.S. Pat. No. 4,561,780 to Ljung et al pertains to path length control as necessary to prevent the starting of the ring laser gyroscope at a point in-between axial modes. This is a more conventional type of path length control. U.S. Pat. No. 4,715,713 to Hutchings discloses a path length adjuster for a ring laser gyro. This invention seeks to strain the laser frame symetrically from the inside of a flexure bore to maintain cavity length. This approach works well for large ring laser gyros that can tolerate a large hole in the center of the frame. The present invention describes a method whereby the frame is strained symetrically from top to bottom through a bolt diameter for smaller ring laser gyros. The present invention relies on laser frame machining symmetry, and can work with ring laser gyros of the smallest size. The Hutchings disclosure can not be applied to laser frames much smaller than 20 cm in path length.
Besides the above, ring laser gyro technology has utilized deformable mirrors and mirror actuators to control path length variations caused by thermal expansion. The requirement exists for a smaller, low cost ring laser gyro and it is not practical or economical to implement a deformable mirror and a mirror actuator of such small dimensions (0.25 in. typical) on such a gyro. Because of all of the above, a new path length control concept is required which will precisely control ring laser gyro pathlength while also reducing cost, parts count and assembly time.